1. Field of the Invention
This invention relates to an exhaust gas recirculation control system and method and an engine control unit for an internal combustion engine, for controlling an exhaust gas recirculation (EGR) rate so as to reduce NOx emission.
2. Description of the Prior Art
Conventionally, an exhaust gas recirculation control system of the above kind was disclosed e.g. in Japanese Patent Registration No. 2503387. The exhaust gas recirculation (hereinafter referred to as xe2x80x9cEGRxe2x80x9d) control system includes an EGR control valve arranged in an EGR passage, for controlling an EGR rate, a NOx sensor arranged in an exhaust passage at a location downstream of a catalyst, for detecting the concentration of NOx in exhaust gases, an air flow meter for detecting the amount of intake air, and a vehicle speed sensor which outputs a pulse signal in synchronism with rotation of a propeller shaft.
In the EGR control system, the total amount of NOx in exhaust gases emitted per unit travel distance (1 km) of the vehicle is calculated based on signals delivered from the NOx sensor, the air flow meter and the vehicle speed sensor, and a ratio of the total amount of NOx emitted thus calculated to a target value is compared with a predetermined value. If the ratio is larger than the predetermined value, i.e. if the calculated total amount of NOx is larger than an allowable limit, the degree of opening of the EGR control valve is increased by a predetermined amount, or in other words, the EGR rate is increased. As a result, the combustion temperature of an air-fuel mixture within the combustion chamber is lowered, whereby the amount of NOx in exhaust gases is reduced. On the other hand, if the above ratio is smaller than the predetermined value, i.e. if the calculated total amount of NOx is smaller than the allowable limit, it is judged that the NOx-eliminating performance of the catalyst is well within the limit thereof, and hence the degree of opening of the EGR control valve is decreased by a predetermined amount so as to secure an increased output of the engine, whereby the EGR rate is decreased.
However, the above conventional EGR control system for an internal combustion engine only calculates the EGR rate for the next control cycle by using the parameter of the total amount of NOx in exhaust gases emitted per unit travel distance, i.e. the total amount of NOx which has already been actually emitted via the catalyst. For this reason, when the NOx-eliminating performance of the catalyst is low in the present control cycle, and NOx is exhausted from the engine at a flow rate far beyond the NOx-eliminating of the catalyst, a large amount of NOx which cannot be treated by the catalyst is emitted into the atmosphere, so that the engine is left in a state of high exhaust emissions until the EGR rate is updated in the next control cycle.
It is an object of the invention to provide an exhaust gas recirculation control system and method and an engine control unit for an internal combustion engine, which is capable of properly controlling an exhaust gas recirculation rate in advance according to the NOx-eliminating performance of a catalyst, thereby reducing exhaust emissions.
To attain the above object, according to a first aspect of the present invention, there is provided an exhaust gas recirculation control system for an internal combustion engine that eliminates NOx in exhaust gases by a catalyst, and recirculates the exhaust gases into an intake system,
the exhaust gas recirculation control system comprising:
operating condition-detecting means for detecting operating conditions of the engine;
catalyst performance-estimating means for estimating NOx-eliminating performance of the catalyst; and
exhaust gas recirculation rate-setting means for setting, in response to the detected operating conditions of the engine and the estimated NOx-eliminating performance, an exhaust gas recirculation rate at which the exhaust gases are recirculated into the intake system.
According to this exhaust gas recirculation control system for an internal combustion engine, the operating conditions of the engine are detected, and the NOx-eliminating performance of the catalyst is estimated. Then, the exhaust gas recirculation rate at which the exhaust gases are recirculated into the intake system is set in response to the detected operating conditions and the estimated NOx-eliminating performance. Since the actual NOx-eliminating performance of the catalyst is estimated and the exhaust gas recirculation rate is controlled, at all times, in response to the estimated NOx-eliminating performance, differently from the prior art, the exhaust gas recirculation control system of the invention is capable of preventing NOx from flowing into the catalyst at a flow rate far beyond the NOx-eliminating performance of the catalyst. This makes it possible to reduce the amount of NOx contained in exhaust gases after treatment by the catalyst, whereby exhaust emissions can be minimized.
Preferably, the exhaust gas recirculation control system further comprises catalyst temperature-estimating means for estimating a temperature of the catalyst based on at least the detected operating conditions of the engine, and the catalyst performance-estimating means estimates the NOx-eliminating performance of the catalyst, according to the estimated temperature of the catalyst.
According to this preferred embodiment, the temperature of the catalyst is estimated based on the operating conditions of the engine, and then the NOx-eliminating performance of the catalyst is estimated according to the estimated temperature of the catalyst. The NOx-eliminating performance of the catalyst depends on the temperature thereof. Therefore, the NOx-eliminating performance of the catalyst can be properly estimated according to the catalyst temperature.
Preferably, the exhaust gas recirculation control system further comprises uneliminated NOx estimate-calculating means for calculating an uneliminated NOx estimate which is defined as an amount of NOx estimated to be emitted from the catalyst without being eliminated thereby, based on the NOx-eliminating performance estimated by the catalyst performance-estimating means, the uneliminated NOx estimate-calculating means calculating a present value of the uneliminated NOx estimate corresponding to a present value of the exhaust gas recirculation rate, a first predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate increased from the present value thereof by a predetermined value, and a second predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, and when the present value of the uneliminated NOx estimate is larger than the second predicted value, the exhaust gas recirculation rate-setting means sets the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, whereas when the present value of the uneliminated NOx estimate is equal to or smaller than the second predicted value and at the same time larger than the first predicted value, the exhaust gas recirculation rate-setting means sets the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate increased from the present value thereof by the predetermined value.
More preferably, the catalyst performance-estimating means includes estimated NOx elimination ratio-calculating means for calculating an estimated NOx elimination ratio to which NOx flowing into the catalyst is estimated to be reduced by the catalyst, according to the estimated temperature of the catalyst, and the uneliminated NOx estimate-calculating means calculates a NOx exhaust estimate which is defined as an amount of NOx estimated to be emitted from the engine and flow into the catalyst, based on the operating conditions of the engine and the exhaust gas recirculation rate, and calculates the uneliminated NOx estimate based on the calculated NOx exhaust estimate and the calculated estimated NOx elimination ratio.
To attain the above object, according to a second aspect of the invention, there is provided an exhaust gas recirculation control system for an internal combustion engine that eliminates NOx in exhaust gases by a catalyst, and recirculates the exhaust gases into an intake system,
the exhaust gas recirculation control system comprising:
an operating condition-detecting module for detecting operating conditions of the engine;
a catalyst performance-estimating module for estimating NOx-eliminating performance of the catalyst; and
an exhaust gas recirculation rate-setting module for setting, in response to the detected operating conditions of the engine and the estimated NOx-eliminating performance, an exhaust gas recirculation rate at which the exhaust gases are recirculated into the intake system.
According to the second aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
Preferably, the exhaust gas recirculation control system further comprises a catalyst temperature-estimating module for estimating a temperature of the catalyst based on at least the detected operating conditions of the engine, and the catalyst performance-estimating module estimates the NOx-eliminating performance of the catalyst, according to the estimated temperature of the catalyst.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
Preferably, the exhaust gas recirculation control system further comprises an uneliminated NOx estimate-calculating module for calculating an uneliminated NOx estimate which is defined as an amount of NOx estimated to be emitted from the catalyst without being eliminated thereby, based on at least the NOx-eliminating performance estimated by the catalyst performance-estimating module, the uneliminated NOx estimate-calculating module calculating a present value of the uneliminated NOx estimate corresponding to a present value of the exhaust gas recirculation rate, a first predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate increased from the present value thereof by a predetermined value, and a second predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, and when the present value of the uneliminated NOx estimate is larger than the second predicted value, the exhaust gas recirculation rate-setting module sets the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, whereas when the present value of the uneliminated NOx estimate is equal to or smaller than the second predicted value and at the same time larger than the first predicted value, the exhaust gas recirculation rate-setting module sets the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate increased from the present value thereof by the predetermined value.
More preferably, the catalyst performance-estimating module includes an estimated NOx elimination ratio-calculating module for calculating an estimated NOx elimination ratio to which NOx flowing into the catalyst is estimated to be reduced by the catalyst, according to the estimated temperature of the catalyst, and the uneliminated NOx estimate-calculating module calculates a NOx exhaust estimate which is defined as an amount of NOx estimated to be emitted from the engine and flow into the catalyst, based on the operating conditions of the engine and the exhaust gas recirculation rate, and calculates the uneliminated NOx estimate based on the calculated NOx exhaust estimate and the calculated estimated NOx elimination ratio.
To attain the above object, according to a third aspect of the invention, there is provided a method of controlling an exhaust gas recirculation rate for an internal combustion engine that eliminates NOx in exhaust gases by a catalyst, and recirculates the exhaust gases into an intake system,
the method comprising the steps of:
detecting operating conditions of the engine;
estimating NOx-eliminating performance of the catalyst; and
setting, in response to the detected operating conditions of the engine and the estimated NOx-eliminating performance, an exhaust gas recirculation rate at which the exhaust gases are recirculated into the intake system.
According to the third aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
Preferably, the method further comprises the step of estimating a temperature of the catalyst based on at least the detected operating conditions of the engine, and the step of estimating the NOx-eliminating performance of the catalyst includes estimating the NOx-eliminating performance of the catalyst, according to the estimated temperature of the catalyst.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
Preferably, the method further comprises the step of calculating an uneliminated NOx estimate which is defined as an amount of NOx estimated to be emitted from the catalyst without being eliminated thereby, based on at least the NOx-eliminating performance estimated in the step of estimating NOx-eliminating performance of the catalyst, the step of calculating an uneliminated NOx estimate including calculating a present value of the uneliminated NOx estimate corresponding to a present value of the exhaust gas recirculation rate, a first predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate increased from the present value thereof by a predetermined value, and a second predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, and when the present value of the uneliminated NOx estimate is larger than the second predicted value, the step of setting an exhaust gas recirculation rate includes setting the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, whereas when the present value of the uneliminated NOx estimate is equal to or smaller than the second predicted value and at the same time larger than the first predicted value, the step of setting an exhaust gas recirculation rate includes setting the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate increased from the present value thereof by the predetermined value.
More preferably, the step of estimating NOx-eliminating performance of the catalyst includes the step of calculating an estimated NOx elimination ratio to which NOx flowing into the catalyst is estimated to be reduced by the catalyst, according to the estimated temperature of the catalyst, and the step of calculating an uneliminated NOx estimate includes calculating a NOx exhaust estimate which is defined as an amount of NOx estimated to be emitted from the engine and flow into the catalyst, based on the operating conditions of the engine and the exhaust gas recirculation rate, and calculating the uneliminated NOx estimate based on the calculated NOx exhaust estimate and the calculated estimated NOx elimination ratio.
To attain the above object, according to a fourth aspect of the invention, there is provided an engine control unit including a control program for causing a computer to carry out exhaust gas recirculation rate control for an internal combustion engine that eliminates NOx in exhaust gases by a catalyst, and recirculates the exhaust gases into an intake system,
wherein the control program causes the computer to detect operating conditions of the engine, estimate NOx-eliminating performance of the catalyst, and set, in response to the detected operating conditions of the engine and the estimated NOx-eliminating performance, an exhaust gas recirculation rate at which the exhaust gases are recirculated into the intake system.
According to the fourth aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
Preferably, the control program causes the computer to estimate a temperature of the catalyst based on at least the detected operating conditions of the engine, and when causing the computer to estimate the NOx-eliminating performance of the catalyst, causes the computer to estimate the NOx-eliminating performance of the catalyst, according to the estimated temperature of the catalyst.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
Preferably, the control program causes the computer to calculate the uneliminated NOx estimate which is defined as an amount of NOx estimated to be emitted from the catalyst without being eliminated thereby, based on at least the NOx-eliminating performance estimated by causing the computer to estimate the NOx-eliminating performance of the catalyst, the control program causing the computer to calculate a present value of the uneliminated NOx estimate corresponding to a present value of the exhaust gas recirculation rate, a first predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate increased from the present value thereof by a predetermined value, and a second predicted value of the uneliminated NOx estimate corresponding to a value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, and when the present value of the uneliminated NOx estimate is larger than the second predicted value, the control program causes the computer to set the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate decreased from the present value thereof by the predetermined value, whereas when the present value of the uneliminated NOx estimate is equal to or smaller than the second predicted value and at the same time larger than the first predicted value, the control program causes the computer to set the exhaust gas recirculation rate to the value of the exhaust gas recirculation rate increased from the present value thereof by the predetermined value.
More preferably, when causing the computer to estimate the NOx-eliminating performance of the catalyst, the control program causes the computer to calculate an estimated NOx elimination ratio to which NOx flowing into the catalyst is estimated to be reduced by the catalyst, according to the estimated temperature of the catalyst, and when causing the computer to calculate the uneliminated NOx estimate, the control program causes the computer to calculate a NOx exhaust estimate which is defined as an amount of NOx estimated to be emitted from the engine and flow into the catalyst, based on the operating conditions of the engine and the exhaust gas recirculation rate, and calculate the uneliminated NOx estimate based on the calculated NOx exhaust estimate and the calculated estimated NOx elimination ratio.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.